Process of manufacturing a magnetic material and magnetic core



Oct. 26, 1948. J. L. SNOEK 2,452,531

PROCESS OF MANUFACTURING A MAGNETIC MATERIAL AND MAGNETIC CORE FiledSept. 19, 1945 o 1 70o g vI000 v FIGE a 1 L 0 7o 700 KH lama FIG.2

l SNOEK H63 JACOB LOUS INVE BY%%M.

ATTORNEY Patented Oct. 26, 1948 PROCESS OF MANUFACTURING A MAGNETICMATERIAL AND MAGNETIC CORE Jacob Louis Snoek, Eindhoven, Netherlands,is. signer to Hartford National Bank & Trust 00., Hartford, Conn, astrustee Application September 19, 1945, Serial No. 617,393 In theNetherlands May 31,1943

Section 1, Public Law 690, August 8, 1946 Patent expires May 31, 1963 19Claims. (Cl. 252-625) This invention relates to the manufacture ofmagnetic cores having a cubic ferrite as a magnetic material, which giverise to small losses at low inductance, even with high frequencies, moreparticularly cores for telegraphy and telephony purposes, for examplefor filter coils, pupin coils, etc.

It is known that magnetic cubic ferrites ordinarily have a high specificelectrical resistance, for example 1000 ohms cm. and higher, and sincewith a resistance of to 100 ohms cm. the eddycurrents are alreadyextremely feeble, such ferrites practically do not exhibit eddy-currentlosses. It appears, however,'that in spite of the eddy-current lossesbeing substantially nil, there may nevertheless occur considerablelosses.

Reference is made to my copending U. S. applications Serial Nos. 616,928filed September 17, 1945, and 617,392 filed September 19, 1945, of whichthe present application is a continuationin-part.

The present invention is based on the recognition of the fact that theselosses are connected with the oxygen contents of ferrite.

With regard to the percentage of oxygen it may be mentioned that it isknown that a ferrite when heated to high temperatures such, for example,as are utilized in its preparatiom'may split off oxygen. In order toavoid such a deficiency of oxygen, the heating in question was carriedout in pure oxygen.

Now, it has been found that, even if a heating required for thepreparation or any other purpose is carried out in pure oxygen; a smalldeficiency of oxygen frequently occurs and that to our surprise such asmall deficiency of oxygen, which may be only a few hundredths ofpercent by weight of the total weight of the ferrite, is highlydisadvantageous for the losses.

Now, the invention consists in that in the manuiacture of a magneticmaterial constituted by a magnetic cubic ferrite having a high specificre sistance, higher than 1000 ohms cm., so high a percentage of oxygenof the ferrite is provided for that the loss factor too in the frequencyrange of from 10 to 100 kllocycles/sec. is less than 0.06. The value tgtis equal to R being the loss resistance measured whilst avoiding theoccurrence of dielectric losses and deduction being made of thedirect-current resistance, L representing the inductance of a coil 2wound on an annular core of the ferrite, and (0 being the annularfrequency.

A percentage of oxygen according to the invention may be obtained indifierent manners,

according to the kind and the composition of the ferrite.

The process, fundamentally consists simply that by a suitable choice ofthe conditions care is taken that the heating temperature required forthe preparation of the ferrite is maintained sumciently low.

With regard to the heating temperature it is observed that, for example,with a ferrite which is prepared by heating an intimate mixture of thesolid oxides constituting the ferrite the temperature will depend on theintensity with which, and the fineness to which the mixture is ground. Avery fine mixture ground for a long time will within a reasonable periodbe capable of yielding a homogeneous product thoroughly reacted at lowertemperatures than a coarser mixture ground for a shorter time; as willbe explained more fully hereinafter, such a complete reaction is ofgreat importance in view of the initial permeability of the finalproduct.

Further, it has been found that in definite cases,

' the sintering temperature is somewhat decreased by decreasing thepercentage of iron oxide of the ferrite.

Although the variation of the conditions under which a ferrite isprepared permits a certain amount of latitude in the heating temperaturerequired, it is frequently not possible in practice to provide for asufficiently high percentage of oxygen already during the preparation ofthe ferrite. According to the invention, such a ferrite having a too lowpercentage of oxygen, which consequently is unsaturated with respect tooxygen, may be caused to absorb oxygen, for example at lowertemperature.

The conditions, more particularly the temperature at which a ferrite iscaused to absorb oxygen, depend on several factors, such as on theamount of oxygen which must be absorbed to obtain a sufiiciently lowvalue for tga, on the degree to which all parts of the ferrite areaccessible to oxygen, on its fineness of grain and hence on itsporosity, and further on the kind and'the composition of the ferriteused.

It has been found that with unvaried oxygen pressure the amount ofoxygen absorbed increases with a decrease of temperature. 0n the otherhand it is necessary to consider the circumstance that the speed ofabsorption of oxygen decreases with a decrease in temperature. Thisspeed further strongly depends on the fineness of grain and the porosityof the ferrite so that in view of the time which would otherwise betaken up by the absorption of oxygen, it is desirable that the ferriteshould be utilized in the fine-grained porous state.

It is observed that to our surprise it was found possible to manufacturea ferrite in a sintered state which is sufficiently compact to serve asa magnetic core whilst maintaining at the same time suificient porosity.

An important factor which must be considered in utilizing the inventionis the initial permeability of the final product since theserviceability of a magnetic core is substantially determined by thevalue of the quotient of the above-defined loss factor tat and theinitial permeability ,u. measured on an annular core. Cores having ahigh initial permeability and low losses are of great importance.Consequently, in the manufacture of such cores one will desire tocombine the steps for obtaining a sufliciently high percentage of oxygenwith steps which are required to obtain a high initial permeability,care having to be taken to see to it that the different steps do notcome into conflict with one another.

It has now been found that for obtaining a high value for the initialpermeability it is desirable to ensure that the ultimate ferriteapproaches as far as possible the state of a single homogeneous phase,i. e. it must be ensured that the ferrite forming initial mixture reactsthoroughly while during cooling the ferrite once produced must beprevented as far as possible from splitting off a second phase. Thelatter case may occur when during cooling the ferrite splits off one ofits constitutive oxides which at high temperature is so-to-saymaintained in an oversaturated solid solution, or when during coolingthe ferrite will fall into its constitutive oxides. If there is a dangerof a second phase being separated, this may be avoided by rapid cooling,although a rapid cooling, more rapid than, for example, at the rate ofC. per minute is as a rule to be avoided, since thus chilling tensionsmay result which are detrimental to the permeability. The mostadvantageous speed of cooling may be easily ascertained experimentallyin each individual case. From the foregoing it further follows thatduring the absorption of oxygen the temperature must preferably remainabove the temperature at which a second phase may be produced.

Further it has been found that, like with other 7 magnetic materials,the initial permeability of the ferrite is at a maximum at a temperaturein the neighborhood of, but less than the Curie point, the Curie pointbeing the temperature at which the magnetic material for all practicalpurposes may be considered non-magnetic; in view thereof a ferritehaving a Curie-point located between 40 C. and 250 C. is highlyadvantageous. A ferrite of this kind is obtained, for example, bycombining zinc ferrite, which has a low Curiepoint, with one or moreferrites having a higher Curie-point, such as nickel ferrite, so that amixed crystal is obtained.

It is also possible to influence the Curie-point of the ferrite byregulating its percentage of iron oxide while, in addition, thepercentage of oxygen may be of influence. The term Curie-point has to beunderstood in this case to mean the temperature at which the ferrite, asregards its initial permeability, changes into a state to be consideredas unmagnetic for practical purposes.

For obtaining a high initial permeability the use of pure raw materialsalso is of much importance.

As previously mentioned, there is a danger of the steps required for theobtainment of a high initial permeability coming into conflict withsteps required for a sufficiently high percentage of oxygen which isdesirable in view of the losses. Thus, for obtaining a high initialpermeability, heating to a high temperature is generally desirable inorder to facilitate the thorough reaction and the formation of ahomogeneous phase. The heating temperature necessary to obtain thehighest possible initial permeability is in most cases, however, suchthat the percentage of oxygen and the possibility of absorption ofoxygen are detrimentally affected by such a heating, the obtainment oflow losses thus being rendered difficult. Consequently, in this case onehas to arrive at a compromise solution.

In my copending U. S. application Serial No. 616,928, filed September17, 1945, I have described a method of manufacturing a homogeneous mixedcrystal ferrite having an oxygen contentat which the coefficient oflosses tgc in a frequency range between 10 kc./sec. and 1000 kc./ sec.is less than 0.06 which is particularly suited for radio purposes. Inorder to provide for such a percentage of oxygen, it is necessary in thepreparation of ferrite, or with a heating for other purposes not toutilize an excessive temperature; due ta the avoidance of a hightemperautre an optimum value for the permeability is frequently notobtained. In this case one has to do with a. compromise, as abovereferred to.

Now, it has been found that, when using higher sintering temperature, itis still possible to obtain such a percentage of oxygen that the lossfactor tg6 through a region extending to less high frequencies is lowerthan 0.06. With higher frequencies the loss factor exceeds this limit.

This material sintered at a higher temperature differentiates from thematerial sintered at a lower temperature amongst others by a coarsergranular structure.

Such a. magnetic material which has a loss factor higher than 0.06 withfrequencies lower than 1000 kilocycles/sec. is not suited to radiopurposes, it is true, but it is still well serviceable for telegraphy ortelephony. Besides, it offers the advantage that in many cases, due tothe higher permissible sintering temperature, a high er initialpermeability is obtained.

Although, as a matter of fact, the initial permeabilities that can beobtained are different, amongst others because one ferrite duringcooling separates a second phase less easily than another and may thusbe brought more-easily into the state which approaches that of a singlehomogeneous phase, with the use of a ferrite of suitable composition ithas in many cases been found possible to make a magnetic material ofwhich the value of is smaller than 0.0001 from 10 to kilocycles/sec.Such a material is excellently suited to telegraphy and telephonypurposes, for example to the manufacture of filter coils, with which itis desired to utilize frequencies of about 10 to 100 kilocycles/sec.Also for pupin coils which as a rule are used with frequencies of from300 to 2000 cycles/sec, such a magnetic material is excellently suited.According to the invention, very good results may be obtained withferrites of which the percentage of iron oxide is less than 50 mol. percent.

As previously mentioned, the serviceability of a magnetic material issubstantially determined by the value of the quotient To explain this,it is observed that the reason why this material constant has such asuitable value for'the judgment of a magnetic material is that in amagnetic circuit having one or more so-called air-gaps the quotient tgsall I III of the effective loss factor time and of the effecin which tgtand pare measured on an annular core.

When-the constant value of the quotient for a ferrite has once beenfixed, for example by measurements on an annular core of the ferrite.the loss factor of another core built up from this material mayconsequently be found by multiphcation of the Constant with theeffective permeability of this core.

A ferrite according to the invention is preferably prepared bycompressing and subsequently sinteringa mixture of the oxidesconstituting the ferrite, or a corresponding mixture of compounds whichupon heating change into oxide.

To obtain a product reacted as thoroughly as possible, it is desirable,as previously mentioned, to start from a ferrite-forming initial mixtureof great fineness and reactivity. In order to obtain a great reactivity,the initial mixture may be ground for a long time and with greatintensity, this grinding being preferably effected to such extent thatan average size of particles smaller than 1 ,u is obtained.

It is alternatively possible to precipitate a solution containing allthe metals constituting the ferrite with the aid of a base and to drythe deposit, obtained, which may in part already exhibit ferritestructure. Subsequently, in order to improve the mouldability, thedeposit is preferably heated to 500 to 700C.

Further, in the preparation of a ferrite it is possible to utilizesintering several times, that is to say that the mixture at firstsintered is pulverized and sintered again. In this case the firstsinterings are effected at lower temperature, during which the mixturedoes not yet show a complete reaction. The product obtained can then beeasily reground to great fineness. A compression of the mixture to bepreliminarily sintered is preferably omitted, also to facilitategrinding. This method of preparation offers the advantage that at leasta well reacted product is obtained, which increases the value of theinitial permeability.

In my copending U. S. application Serial No. 616,928, filed September17, 1945, of which the present application is a continuation-in-part, Ihave described and claimed homogeneous mixed crystal ferrites having anoxygen content at which the coemcient of losses too in the frequencyrange between 10 kc./sec. and 1000 kcJsec. is less than 0.06. Coreshaving such magnetic materials are excluded from the exclusive rightsnow claimed. It is still mentioned that the expression magnetic core inthe present invention covers not only a core arranged inside a coil butas a rule parts of electro-magnetic constructions which are utilized inview of their magnetic properties, for example also parts for magneticscreening.

In order that the invention may be more readily understood it will nowbe described with reference to the following examples and theaccompanyinz drawing in which:

Fig. 1 shows the relationship between the quotient of the coeflicient oflosses and permeability p with frequency for Example I,

Fig. 2 shows the relationship between the quotient of the coeflicient oflosses and P rmeability with frequency for Example II; and

Fig. 3 shows a core composed essentially of magnetic ferrite materialaccording to the invention.

Referring to the drawing, I have shown a sample ring core ill of ahomogeneous mixed crystal ferrite which is provided with an oxygencontent at which the coefficient of losses tat is less than 0.06 in afrequency range between 10 and kc./sec. and greater than 0.06 at afrequency between 100 and 1000 kcjsec.

Example I An intimate mixture of pure magnesium oxide, zinc oxide andiron oxide in a molecular ratio of 26.5:26.5:47 is ground during 3 hoursin an iron hurling mill. A ring of 3 cm. in diameter and 4 x 4 mm. insection is moulded from the mixture under a. pressure of 4 tons/cm. withthe use of water as a plastifying and binding agent. This ring is heatedin oxygen to 1300 C. during 1 hour, followed by cooling in oxygen at a.rate of about 3 C. per minute. The initial permeability amounts to 350.The values of are plotted in Figure 1 in dependence on frequency, curvea. From this curve it follows that with frequencies higher than 800kilocycles/sec. the value for tga becomes higher than 0.06.

If the heating is effected at 1400 C. instead of 1300" 0., one obtainsthe values of curve b shown in Figure 1. In this case the initialpermeability amounts to 525. With frequencies higher than 350kilocycles/sec. i578 becomes higher than 0.06.

Example If A mixture of technical copper oxide, zinc oxide and ironoxide, in a. molecular ratio of calculated on the pure oxides, which isadditioned by 1% by weight of brownstone, is ground during 3 hours andsubsequently moulded to form a ring in the manner described in ExampleI.

This ring is sintered in oxygen at 1050 C. during 1 hour andsubsequently slowly cooled down to 600 0., which temperature ismaintained during 14 hours, all this in oxygen, whereafter furthercooling takes place. The copper-zincferrite core has an initialpermeability of 385. The values of are plotted in Figure 2 in dependenceof frequency. From this figure it follows that with frequencies higherthan 900 kilocycles/sec. the value for tat becomes higher than 0.06.

For the purpose of defining the terms "ferrite," "mixed crystal," and"soft magnetic materials, the following definitions will be employed inconnection with the above-noted terms as used throughout thespecification and in the appended c aims.

A "ferrite is a crystalline material which is a compound of the reactionproduct of a metal oxide and iron oxide having the empirical formulaMFezOl wherein M represents a bivalent metal. This material may also bedefined as a metallic salt of the hypothetical acid HzFeaOa.

A mixed crystal" ferrite is a ferrite material comprising two or moreferrites as hereinbefore defined which are chemically combined togetherto form a single homogeneous crystalline compound.

The term "soft" magnetic material means magnetic material having a lowremanence and a low coercivity when the applied inductive field isremoved from the material.

Within the scope of the definitions noted above, I have described myinvention with specific examples and methods of execution, which,however, will suggest other obvious modifications to those skilled inthe art without departing from the spirit and scope of my invention.

What I claim is:

1. A soft ferromagnetic core material having low magnetic losses in afrequency range between 10 kc./sec. and 100 kc./sec. and increasingmagnetic losses above 100 kc./sec. and which is particularly suited forcores, inductance coils and the like employed in telephony andtelegraphy circuits operating within said range of frequencies,consisting essentially of a cubic homogeneous mixed crystal ferriteconstituted by a plurality of ferrites, said mixed crystal ferritehaving a specific resistivity greater than about 1000 ohm-cm. and havingan oxygen content at which the coefiicient of losses tg6 thereof is lessthan about 0.06 in the range of frequencies between about 10 kc./sec.and 100 kc./sec. and greater than 0.06 at a frequency between about 100kc./sec. and about 1000 kc./sec.

2. A soft ferromagnetic core material having low magnetic losses in afrequency range between 10 kc./sec. and 100 kc./sec. and increasingmagnetic losses above 100 kc./sec. and which is particularly suited forcores, inductance coils and the like employed in telephony andtelegraphy circuits operating within said range of frequencies,consisting essentially of a cubic homogeneous mixed crystal ferriteconstituted by a plurality of ferrites, said mixed crystal ferritehaving a specific resistivity greater than about 1000 ohm-cm. and havingan oxygen content at which the coefiicient of losses tga of said mixedcrystal ferrite is less than about 0.06 in the range of frequenciesbetween about 10 kc./sec. and 100 kc./sec. and greater than 0.06 at afrequency between about 100 kc./sec. and about 1000 kc./sec., said mixedcrystal ferrite having a Curie point between 40 C. and 250 C.

3. A soft ferromagnetic core material having low magnetic losses in afrequency range between 10 kcJsec. and 100 kc./sec. and increasingmagnetic losses above 100 kc./sec. and which is particularly suited forcores, inductance coils and the like. employed in telephony andtelegraphy circuits operating within said range of frequences.consisting essentially of a cubic homogeneous mixed crystal ferriteconstituted by zinc ferrite and a second ferrite having a Curie pointgreater than that of said zinc ferrite, said mixed crystal ferritehaving a specific resistivity greater than about 1000 ohm-cm. and anoxygen content at which the coeflicient of losses tat of said mixedcrystal ferrite is less than about 0.06 in the range of frequenciesbetween about 10 kc./sec. and 100 kc./sec. and greater than 0.06 at afrequency between about 100 kc./sec. and about 1000 kc./sec., said mixedcrystal ferrite having a Curie point between about 40 C. and 250 C.

4. A soft ferromagnetic core material having low magnetic losses in afrequency range between 10 kc./sec. and 100 kc./sec. and increasingmagnetic losses above 100 kc./sec. and which is particularly suited forcores, inductance coils and the like employed in telephony andtelegraphy circuits operating within said range of frequencies,consisting essentially of a cubic homogeneous mixed crystal ferriteconstituted by zinc ferrite and a second ferrite having a Curie pointgreater than that of said zinc ferrite, said mixed crystal ferritehaving a specific resistivity greater than about 1000 ohm-cm. and anoxygen content at which the quotient of the coefficient of losses tgfidivided by the initial permeability a of said mixed crystal ferrite inthe frequency range between about 10 kc./sec. and 100 kc./sec. is lessthan about 0.0001 and having a loss factor tgt which is greater than0.06 at a frequency between about 100 kc./sec. and about 1000 kc./sec.,said mixed crystal ferrite having a Curie point between about 40 C. and250 C.

5. A soft ferromagnetic core material having low magnetic losses in afrequency range between 10 kc./sec. and 100 kc./sec. and increasingmagnetic losses above 100 kc./sec. and which is particularly suited forcores, inductance coils and the like employed in telephony andtelegraphy circuits operating within said range of frequencies,consisting essentially of a cubic homogeneous mixed crystal ferriteconstituted by zinc ferrite and magnesium ferrite, said mixed crystalferrite having a specific resistivity greater than about 1000 ohm-cm.and an oxygen content at which the quotient of the coefllcient of lossestgt divided by the initial permeability a of said mixed crystal ferritein the frequency range between about 10 kc./sec. and 100 kc./sec. isless than about 0.0001 and having a loss factor fall which is greaterthan 0.06 at a frequency between about 100 kc./sec. and about 1000kc./sec., said mixed crystal ferrite having a Curie point between about40 C. and 250 C 6. A soft ferromagnetic core. material having lowmagnetic losses in a frequency range between 10 kc./sec. and 100kc./sec. and increasing magnetic losses above 100 kc./sec. and which isparticularly suited for cores, inductance coils and the like employed intelephony and telegraphy circuits operating within said range offrequencies, consisting essentially of a cubic homogeneous mixed crystalferrite constituted by zinc ferrite and copper ferrite, said mixedcrystal ferrite having a specific resistivity greater than about 1000ohm-cm. and an oxygen content at which the quotient of the coeflicientof losses tgt divided by the initial permeability ,u of said mixedcrystal ferrite in the frequency range between about kc./sec. and 100kc./sec. is less than about 0.0001 and having a loss factor tga which isgreater than 0.06 at a frequency between about 100 kc./sec. and about1000 kc./sec., said mixed crystal ferrite having a Curie point betweenabout 40 C. and 250 C.

'7. A soft ferromagnetic core material having low magnetic losses in afrequency range between 10 kc./sec. and 100 kc./sec. and increasingmagnetic losses above 100 kc./sec. and which is particularly suited forcores, inductance coils and the like employed in telephony andtelegraphy circuits operating within said range of frequencies,consisting essentially of a cubic homogeneous mixed crystal ferriteconstituted by magnesium ferrite and zinc ferrite having oxidecomponents in an amount equivalent to approximately 26.5 mol. per centof magnesium oxide, 26.5 mol. per cent of zinc oxide, and 47 mol. percent of iron oxide, said mixed crystal ferrite having a specificresistivity greater than about 1000 ohm-cm. and an oxygen content atwhich the quotient of the coefficient of losses ty 6 divided by theinitial permeability a of said material in the frequency range betweenabout 10 kc./sec. and 100 kc./sec. is less than about 0.0001 and havinga loss factor to a which is greater than 0.06 at a frequency betweenabout 100 kc./sec. and about 1000 kc./sec., said mixed crystal ferritehaving a Curie point between about 40 and 250 C.

8. A soft ferromagnetic core material having low magnetic losses in afrequency range between 10 kc./sec. and 100 kc./sec. and increasingmagnetic losses above 100 ire/sec. and which is particularly suited forcores, inductance coils and the like employedin telephony and telegraphycircuits operating within said range of frequencies, consistingessentially of a cubic homogeneous mixed crystal ferrite constituted bycopper ferrite and zinc ferrite having oxide components in an amountequivalent to approximately 20.7 mol. per cent of copper oxide, 31.6

of the coefficient of losses tg 6 divided by the initial permeability aof said material in the frequency range between about 10 kc./sec. and100 kc./sec. is less than about 0.0001 and having a loss factor ty 6which is greater than 0.06 at a frequency between about 100 kc./sec. andabout 1000 kc./sec., said mixed crystal ferrite having a Curie pointbetween about 40 C. and 250 C.

9. The method of manufacturing a soft ferromagnetic core material havinglow magnetic losses in a frequency range between 10 kc./sec. and 100kc./sec. and increasing magnetic losses above 100 kc./sec. which isparticularly suited for cores, inductance coils and the like employed intelephony and telegraphy circuits operating within said range offrequencies, comprising the steps of heating a mixture of a first cubicferrite and a second cubic ferrite to a temperature between about 1000C. and about 1400 C. in an oxygen-controlling atmosphere to produce ahomogeneous mixed crystal ferrite, and regulating the oxygen content insaid mixed crystal ferrite to produce a mixed crystal ferrite materialhaving a coefficient of losses to 6 which is less than about 0.06 in therange of frequencies between 10 about 10 kc./sec. and kc./sec. andgreater than 0.06 at a frequency between about 100 kc./sec. and about1000 kc./sec.

10. The method of manufacturing a soft ferromagnetic core materialhaving low magnetic losses in a frequency range between 10 kc./sec. and100 kc./sec. and increasing magnetic losses above 100 kc./sec. which isparticularly suited for cores, inductance coils and the like employed intelephony and telegraphy circuits operating within said range offrequencies, comprising the steps of heating a mixture of a first cubicferrite with a, second cubic ferrite in an oxygen atmosphere to atemperature between about 1000 C. and about 1400 C. to produce ahomogeneous mixed crystal ferrite, and regulating the oxygen content insaid mixed crystal ferrite to produce a mixed crystal ferrite materialhaving a coefficient of losses to 6 which is less than about 0.06 in arange of frequencies between about 10 kc./sec. and 1000 kc./sec. andgreater than 0.06 at a frequency between about 100 kc./sec. and about1000 kc./sec.

11. The method of manufacturing a soft ferromagnetic core materialhaving low magnetic losses in a frequency range between 10 ire/sec. and100 kc./sec. and increasing magnetic losses above 100 kc./sec. which isparticularly suited for cores, inductance coils and the like employed intelephony and telegraphy circuits operating within said range offrequencies, comprising the steps of pulverizing a mixture of a firstcubic ferrite and a second cubic ferrite to an average grain size ofless than about 1 micron, heating the said mixture to a temperaturebetween about 1000 C. and about 1400 C. in an oxygen atmosphere toproduce a homogeneous mixed crystal ferrite, regulating the oxygencontent in said mixed crystal ferrite to produce a mixed crystal ferritehaving a coefficient of losses tg 6 which is less than about 0.06 in afrequency range between about 10 ire/sec. and 100 kc./sec. and greaterthan 0.06 at a frequency between about 100 ire/sec. and about 1000kc./sec., and cooling the mixed crystal ferrite material at atemperature rate less than about 10 C. per minute in an oxygenatmosphere to maintain the said oxygen content in said material.

12. The method of manufacturing a soft ferromagnetic material having lowmagnetic losses in a frequency range between 10 kc./sec. and 100Ira/sec. and increasing magnetic losses above 100 kc./sec. which isparticularly suited for cores, inductance coils and the like employed intelephony and telegraphy circuits operating within said range offrequencies, comprising the steps of heating a mixture of a first cubicferrite and a second cubic ferrite to a temperature less than 1000 C. inan oxygen atmosphere, pulverizing the so heated mixture to a fine grainsize, heating the pulverized mixture to a temperature between about 1000C. and about 1400 C. in an oxygen atmosphere to produce a homogeneousmixed crystal ferrite, regulating the oxygen content of said mixedcrystal ferrite to produce a mixed crystal ferrite material having acoefficient of losses ty 6 which is less than about 0.06 in a frequencyrange between about 10 kc./sec. and 100 kc./sec. and greater than 0.06at a frequency between about 100 kc./sec. and about 1000 kc./sec., andcooling said mixed crystal ferrite material at a temperature rate lessthan 10 0. per minute in an oxygen atmosphere to maintain the saidoxygen content in said material.

13. The method of manufacturing a soft ferrol1 magnetic core materialhaving low magnetic losses in a frequency range between kc./s ec. and100 kc./sec. and increasing magnetic losses above 100 kc./sec. which isparticularly suited for cores, inductance coils and the like employed intelephony and telegraphy circuits operating within said range offrequencies, comprising the steps of heating a mixture of zinc ferriteand a second ferrite having a Curie point greater than that of said zincferrite in a proportion to produce a homogeneous mixed crystal ferritehaving a Curie point between 40 C. and 250 C. to a temperature less than1000" C. in an oxygen atmosphere, pulverizing the so heated mixture to afine grain size, heating the pulverized mixture to a temperature betweenabout 1000 C. and 1400 C. in an oxygen atmosphere to produce ahomogeneous mixed crystal ferrite, regulating the oxygen content of saidmixed crystal ferrite to produce a mixed crystal ferrite material havinga coeilicient of losses tg 6 which is less than about 0.06 in afrequency range between about -10 kc./sec. and 100 kc./sec. and greaterthan 0.06 at a frequency between about 100 kc./sec.

. and about 1000 kc./sec., and cooling said mixed crystal ferritematerial in an oxygen atmosphere at a temperature rate less than about10 C. per minute to maintain the said oxygen content and to obtain ahomogeneous mixed crystal ferrite having a Curie point between 40 C. and250 C.

14. The method of manufacturing a soft ferromagnetic core materialhaving low magnetic losses in a frequency range between 10 kc./sec. and100 kc./sec. and increasing magnetic losses above 100 kc./sec. which isparticularly suited for cores, inductance coils and the like employed intelephony and telegraphy circuits operating within said range offrequencies, comprising the steps of heating a mixture of zinc ferriteand magnesium ferrite in an oxygen atmosphere to a temperature less than1000 C., pulverizing th so heated mixture to a fine grain size, heatingthe pulverized mixture to a temperature of about 1300" C. in an oxygenatmosphere to produce a homogeneous mixed crystal ferrite, regulatingthe oxygen content of said mixed crystal ferrite to produce a mixedcrystal ferrite material having a coeflicient of losses ty 6 which isless than about 0.06 in a frequency range between about 10 kc./sec. and'100 kc./sec. and greater than 0.06 at a frequency between about 100kc./- sec. and about 1000 kc. /sec., and cooling the mixed crystalferrite material at a temperature rate less than about 10 C. per minuteto maintain the said oxygen content in said material.

15. The method of manufacturing a soft ferromagnetic core materialhaving low magnetic losses in a frequency range between 10 kc./seo. and100 kc./sec. and increasing magnetic losses above 100 kc./sec. which isparticularly suited for cores, inductance coils and the like employed intelephony and telegraphy circuits operating within said range offrequencies, comprising the steps of heating a mixture of zinc ferriteand copper ferrite in an oxygen atmosphere to a temperature less than1000 C., pulverizing the so heated mixture to a fine grain size, heatingthe pulverized mixture to a temperature of about 1050 C. in an oxygenatmosphere to produce a homogeneous mixed crystal ferrite, regulatingthe oxygen content of said mixed crystal ferrite to produce a mixedcrystal ferrite having a coemcient of losses to 6 of the mixed crystalferrite which is less than about 0.06 in a frequency range between about10 kc./sec. and 100 kc./sec.

and greater than 0.06 at a frequency between I about kc./sec. and about1000 kc./sec. and cooling the mixed crystal ferrite material at atemperature rate less than about 10 C. per minute to maintain the saidoxygen content in said material.

16. The method of manufacturing magnetic core material, comprising thesteps of intimately mixing pure powdered magnesium oxide, pure powderedzinc oxide, and pure powdered iron oxide in the molecular ratio of26.5:26.5:5:47, pulverizing said intimate mixture to a grain sizesmaller than about 1 micron, compressing said mixture into a core undera pressure of approximately 4 tons/cmF, heating said core to atemperature of about 1300 C. for about one hour in an oxygen atmosphere,and cooling said core at a temperature rate of about 3 C. per min. in anatmosphere of pure oxygen to allow said heated core to absorb oxygen sothat the coefficient of losses ty 6 is less than about 0.06 atfrequencies greater than about 10 kc./sec. and less than about 100kc./sec. and the initial permeability is about 350.

17. The method of manufacturing magnetic core material, comprising thesteps of intimately mixing pure powdered magnesium oxide, pure powderedzinc oxide, and pure powdered iron oxide in the molecular ratio of26.5:26.5:47, pulverizing said intimate mixture to a grain size smallerthan about 1 micron, compressing said mixture into a core under apressure of approximately 4 tons/cm. heating said core to a temperatureof about 1400 C. for about one hour in an oxygen atmosphere, andcooling'said core at a temperature rate of about 3 C. per min. in anatmosphere of pure oxygen to allow said heated core to absorb oxygen sothat the coeflicient of losses ty 6 is less than about 0.06 atfrequencies greater than about 10 kc./sec. and less than about 100kc./sec. and the initial permeability is about 525. I

18. The method of manufacturing magnetic core material, comprising thesteps of intimately mixing pur powdered copper oxide, pure powdered zincoxide, and pure powdered iron oxide in the molecular ratio of20.7:31.6:47.7, pulverizing said mixture to a grain size smaller thanabout 1 micron, compressing said mixture into a core under a pressure ofabout 4 tons/emf, sintering said core in an oxygen atmosphere at atemperature of about 1050 for approximately one hour, slowly coolingsaid core in an oxygen atmosphere to a temperature of about 600 0.,maintaining the core at about 600 C. for approximately 14 hours in anoxygen atmosphere, and slowly cooling said core in an oxygen atmosphereso that the quotient of the coefiicient of losses E p is less than about0.0001 at frequencies between about 10 kc./sec. and 100 kc./sec. and theinitial permeability is about 385.

19. The method of manufacturing a soft ferromagnetic core materialhaving low magnetic losses in a frequency range between 10 kc./sec. and100 kc./sec. and increasing magnetic losses above 100 kc./sec. which isparticularly suited for cores, inductance coils and the like employed intelephony and telegraphy circuits operating within the said range offrequencies, comprising the steps of heating a first metal oxide andiron oxide forming a first cubic ferrite and a second metal oxide andiron oxide forming a second cubic ferrite to a temperature between 1000"C. and 1400 C. in an oxygen controlling STATES PATENTS atmosphere toproduce a homogeneous mixed Number Name Date crystal ferrite, andregulating the oxygen con- 1,647,737 Legg Nov. 1, 1927 tent in saidmixed crystal ferrite to produce a 6 1,919,806 Schulz July 25, 1933mixed crystal ferrite material having a coem- 1,946,964 Cobb Feb. 13,1934 cient of losses to 8 which is less than 0.06 in the 1,976,230 Katoet a1 Oct. 9, 1934 range of frequencies between about 10 kc./sec. and100 kc./sec. and greater than 0.06 at a. fre- OTHER REFERENCES quencybetween about 100 kc./sec. and 1000 M81101, Comprehensive Treatise onIn- 1 organic and Theoretical Chemistry," Longmans, Green and Co., NewYork, 1932, vol. XII, pages JACOB LOUIS SNOEK. 775-777 and 785.

REFERENCES CITED The following references are of record in the file ofthis patent:

